1. Field of the Invention
The present invention relates to an optical disc playback apparatus, and particularly, to an optical disc playback apparatus performing defocus adjustment of an objective lens.
2. Description of the Background Art
A certain optical disc playback apparatus performs defocus adjustment of an objective lens. For example, Japanese Patent Laying-Open No. 8-339550 discloses the following control method in its description of a conventional technique.
Upon instruction of playing back an optical disc, a control circuit causes a laser diode of an optical pickup to emit light, and thereafter drives a focus actuator to move an objective lens in a prescribed moving range. This moving range is set in advance so that the position of the objective lens corresponding to the focal point of a light beam is present within the moving range.
In parallel, the control circuit performs focus pull-in. More specifically, the control circuit monitors a focus error signal that is detected while the objective lens is moved, and when the objective lens reaches the position corresponding to the focal point, it enters a closed loop-control mode having the focus error signal as a control signal.
Next, the control circuit actuates a spindle motor. When a rough CLV mode is attained, where adjustment allowable range of linear velocity is slightly broader than in a normal playback mode, the control circuit performs an offset adjustment so that the center of the amplitude of the tracking error signal matches with the reference voltage level of servo.
Next, it performs tracking pull-in. After performing tracking pull-in, it performs defocus adjustment to maximize an optimum jitter point or information signal amplitude.
After performing defocus adjustment, it performs AGC (Auto Gain Control) to optimize gains of focus servo and tracking servo, and then data is read.
As above, by performing defocus adjustment of the objective lens so that the jitter is optimized, errors in reading an optical disc can be reduced.
On the other hand, in a seek operation where a light beam crosses tracks, it is known that alternating current components in synchronization with the track-crossing are superimposed on a focus error signal. Such alternating current components occur due to variations in the amplitude among signals respectively output from divided portions of a four-divided photodetector, and also because the focus error signal is generated by astigmatism method. The alternating current components are referred to as T/F crosstalk.
Therefore, even when the defocus amount is adjusted so that the jitter amount is optimized, the focus is lost due to the effect of T/F crosstalk during the seek operation. Thus, the precision in counting the number of tracks being crossed is degraded. As a result, the light beam cannot reach the target track, failing to perform the seek operation.